Systems of interconnected electronic units, for example wireless communication systems, typically comprise interconnected units such as transmitters, receivers, power supply units, fan control units etc. that need to communicate on the data link layer, i.e. layer 2 of the Open Systems Interconnection (OSI) model. For example, such systems, here called master-slave systems, may be characterized by one controlling and a flexible number of controlled units often connected to each other by means of a bus topology. A bus topology has a good scalability and is typically cost effective when considering larger systems.
Examples of protocols that are used in the link layer include the high-level data link control (HDLC) protocol, defined in the International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) standard ISO/IEC13239 and the Ethernet protocol which also is used for other topologies (ring, star, tree and mesh).
In order to enable a controller to individually control a specific unit, an addressing mechanism is essential. Examples of this are the address assignment procedures in the Iuant interface defined by the third generation partnership project (3GPP) in TS 25.462 and the media access control (MAC) addresses used in Ethernet based systems.
One approach to assign unique addresses in HDLC based system is to differentiate between types of units. For example, a power supply unit uses one fixed address and a fan control unit another. However, such an approach does not work if the controller has to control more than one unit of the same type. Another approach is to assign a unit an address by means of, e.g., manually operated dipswitches. These approaches can also be combined. A unit knows its type and can also read the dipswitches to calculate its unique address. A control system that is aware of the addressing rules can communicate with all units in the system as long as the unit addressing is unique.
Ethernet based systems uses MAC addressing, typically in combination with transport control protocol/internet protocol (TCP/IP), which secures that all units at the time of manufacturing are assigned a worldwide unique address. These protocols were developed to implement communication ability for complex systems and are thus not cost optimized to achieve communication within a master-slave system with one controlling unit and a number of controlled units.
One drawback with these master-slave systems, i.e. systems that are less complex than the typical Ethernet/TCP/IP systems, is that there is no built in mechanism to provide information to the controller where a controlled unit is physically located. For small systems with few units placed in near proximity to each other this is not a major problem. However for larger systems where the controlled units are located at geographically different locations it may be time consuming and costly to find and replace a failing unit. For a bus oriented system it is also cumbersome to configure added units in a system extension scenario, because of address dependencies of already connected units.
Another drawback with these master-slave systems becomes apparent when a specific unit (of the same type) is providing a specific service for the controller of another system. Then the physical placement and identity is critical to provide the correct functionality. An example of this is cascaded remote electrical tilting (RET) antennas operating according to 3GPP TS 25.462 (i.e. the Iuant interface). In this case the controller must be configured to know a unique identifier for each specific unit and also to know which specific service, in another system, it is used. If this configuration is done manually, e.g. by use of dipswitches, there is a risk of entering errors with the result that the system behavior will not be the wanted one. An automated design to solve the problem can be complex in its nature and thus costly to develop and maintain. Furthermore, such a design cannot be generic because it must be based on the analyzing of whether the system works as intended, if it was correctly configured, or if it is not working as intended, how the system shall be reconfigured to perform properly.
A further drawback with systems involving dipswitches is that the manual setting of the dipswitch must match how the controller is configured, in order to enable successful communication. If two units are configured with the same dipswitch setting the controller cannot communicate with any of the two units. Another drawback is that dipswitches may be costly and take critical front space on units.
For other units than 3GPP Iuant compliant units it is possible to perform address assignment that avoids physical dipswitches. For example, each type of a controlled unit may be assigned a predefined address range, which together with a number of extra address bits supplied by a hub unit (to which each controlled unit must be connected), enables the controlled unit to calculate its unique address and thereby be able to respond to messages addressed to the unit.
A drawback with such an approach is that the controller unit software needs to be configured to know to which port in the hub a unit is connected to in order to be able to calculate the expected HDLC address the unit will respond to. Thus, if the unit is not connected to the hub in the same way as the control software is configured, the communication will fail. Moreover, it is awkward to expand such a system to more than one hub unit because, when the same type of unit is connected to the corresponding connector port with the same address bits on a second hub unit, the addresses will collide and make the communication impossible with these units.